Process for tantalliding and niobiding base metal compositions

ABSTRACT

A TANTALLIDE OR NIOBIDE COATING IS FORMED ON SPECIFIED BASE METAL COMPOSITIONS BY MAKING THE BASE METAL THE CATHODE JOINED THROUGH AM EXTERNAL ELECTRICAL CIRCUIT TO A TANTAUM OR NIOBIUM ANODE IN AN ELECTRIC CELL HAVING A SPECIFIED FUSED SALT ELECTROLYTE AT A TEMPERATURE OF AT LEAST 900*C., BUT BELOW THE MLETING POINT OF THE METAL COMPOSITION. SUCH A COMBINATION IS A SELF-GENERATING CELL PRODUCING ELECTRIC BUT AN EXTERNAL E.M.F. MAY BE IMPRESSED PROVIDING THE CURRENT DENSITY DOES NOT EXCEED 10 EMPERS/CM.2. THE PROCESS IS USEFUL IN MAKING TIGHT ADHERENT COATINGS COMPOSED OF TANTALUM OR NIOBIUM AND THE BASE METAL ON THE SURFACE OF THE SUBSTRATE.

United States Patent No. 838,636, July 2, 1969. This application Oct.15,

1971, Ser. No. 189,763 The portion of the term of the patent subsequentto May 26, 1987, has been disclalmed Int. Cl. C23b 5/30, 5/48 US. Cl.204-39 11 Claims ABSTRACT OF THE DISCLOSURE A tantallide or niobidecoating is formed on specified base metal compositions by making thebase metal the cathode joined through an external electrical circuit toa tantalum or niobium anode in an electric cell having a specified fusedsalt electrolyte at a temperature of at least 900 C., but below themelting point of the metal composition. Such a combination is aself-generating cell producing electricity, but an external E.MF. may beimpressed providing the current density does not exceed amperes/cm. Theprocess is useful in making tight adherent coatings composed of tantalumor niobium and the base metal on the surface of the substrate.

This is a continuation of application Ser. No. 838,636, filed July 2,1969, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method formetalliding a based metal composition. More particularly, this inventionis concerned with a process for tantalliding and niobiding a base metalcomposition in a fused salt bath.

It is knovm that tantalum and niobium can be electrodeposited at 650 C.to 850 C. on certain metal compositions to form a firmly adherent layerof tantalum or niobium joined to the metal composition by ametal-tometal bond by electrodeposition in a fused salt bath. Thismethod also requires that the molten fluorides must contain at least oneof the fluorides of the group potassium, rubidium, or cesium.

It has been found, however, that these prior art proccesses have anumber of shortcomings which substantially limit their usefulness. Forexample, it has been found that the rate of diffusion and alloying is afunction of the bath temperature. In fact, at lower temperatures, 850 C.and below, the process is principally one in which the tantalum andniobium is plated onto the substrate metal with very little or noalloying taking place. Above 850 C., on the other hand, the displacementand volatilization of the potassium, rubidium and cesium in the moltenfluorides introduces considerable diificulties. Potassium, perhaps,raises the most difficult problems as it volatilizes at 850 C. Anyattempts to operate the bath above 850 C. results in the presence ofpotassium vapors which combine either with the substrate metals or themetals of other fluorides in the bath. Hence, these prior art processescould not be operated at temperatures at which substantial diffusioncoatings of tantalum and niobium, as opposed to electrodeposited layersof these metals, are formed.

We have now discovered that uniform, tough, adherent diffusion coatingsof tantalum or niobium can be formed on certain metal compositionsemploying certain alkali and alkaline earth fluoride molten salts in thecomplete or substantial absence of the fluorides of potassium, rubidium,or cesium, at temperatures in excess of 850 C. This electrodeposition oftantalum and niobium and sub- 3,814,673. Patented June 4, 1974 sequentformation of diffusion coatings is possible if certain critical stepsare taken to insure the substantial absence of oxygen and oxide salts inthe fused salt bath.

In accordance with the process of this invention, the tantalum orniobium metal is employed as the anode and is immersed in a fused saltbath composed essentially of a member of the class consisting of thealkali metal fluorides of lithium and sodium and mixtures thereof andmixtures of the alkali metal fluorides with magnesium, calcium,strontium or barium fluoride and containing from 0.01-5 mole percent oftantalum or niobium fluoride.

The cathode employed is the base metal upon which the diffusion coatingis to be made. We have found that such a combination is an electric cellin which an electric current will be generated when an electricalconnection, which is external to the fused bath, is made between thebase metal cathode and the anode. Under such conditions, the metal ofthe anode dissolves in the fused salt bath and the metal ions aredischarged at the surface of the base metal cathode where they form adeposit of tantalum or niobium which immediately diffuses into andreacts with the base metal to form a metallide coating.

The alkali metal fluorides which can be used in accordance with theprocess of this invention includes the fluorides of lithium and sodium,the mixtures thereof.

Lithium fluoride is preferred, however, because of its lower reactivityat temperatures above 850 C. Eutectic mixtures of lithium and sodiumfluoride can often be used, however, especially at the lower operatingtemperatures of the process.

Mixtures of the all lithium and sodium alkali metal fluorides withmagnesium, calcium, strontium, or barium fluoride can also be employedas components of the molten salts in the process of this invention.Magnesium fluoride does not always function as an inert component,however, since it sometimes permits the incorporation of small amountsof magnesium in the diflfusion coating, and this is not alwaysdesirable.

The chemical composition of the fused salt bath is critical for optimummetalliding results. The starting salt should be as anhydrous and freeof all impurities as is possible or should be easily dried or purifiedby simply heating during the fusion step. The process must be carriedout in the substantial absence of oxygen since oxygen interferes withthe process by forming tantalum or niobium oxides and thereby preventinga coherent dififusiou coating of tantalum or niobium from being formedon the base metal cathode. Thus, for example, the process can be carriedout in an inert gas atmosphere. By the term substantial absence ofoxygen it is meant that neither atmospheric oxygen nor oxides of metalsare present in the fused salt bath. The best results are obtained bystarting With reagent grade salts and by carrying out the process in aninert gas atmosphere, for example, in an atmosphere of argon, helium,neon, krypton or xenon.

We have sometimes found that even commercially available reagent gradesalts must be purified further in order to operate satisfactorily in ourprocess. This purification can be readily done by utilizing scrap metalarticles as the cathodes and carrying out the initial metalliding runswith or without additional applied voltage, thereby plating out andremoving from the bath those impurities which interfere with theformation of high quality metallide coatings.

We have found that in order for the electrolysis cell of this process towork properly and to form metallide coatings which are bright and smoothrather than dull and rough due to the formation of oxidized surfaces itis necessary to reduce the oxide content of the salt to an extremely lowlevel, i.e., about a few parts-per-million,

and to maintain an inert atmosphere over the salt at all times toprevent re-contamination of the salt with oxygen. T antallided andniobided diffusion coatings can be made on some metal surfacessuch asnickel and iron into which tantalum and niobium readily difiusein thepresence of considerable oxygen content in the salt, i.e., a fewparts-per-thousand, but the surfaces are usually dull andmicroscopically rough due to surface oxidation. Such coatings usuallyneed to be slightly thicker than the bright smooth coatings to givecomparable corrosion resistances. In the diffusion of tantalum intometals such as chromium and complex alloys where diffusion is slow, itis much more desirable and often critical that the oxide content of thebaths be extremely low. The oxygen can be removed from the fused saltbath by employing a carbon anode and running the bath as an electrolyticcell to remove the oxides and oxygen by means of the carbon anode. Wehave also found that the last traces of oxygen and oxides can be removedfrom the fused salt bath by maintaining the fused salt bath under aninert atmosphere and placing in the bath, strips or chips of tantalum orniobium for a period of time until the strips or chips upon removal fromthe bath showed no evidence of pitting or other deterioration of theglossy, shiny surface of the metal due to the reaction of the tantalumor niobium with oxygen. Such strips of metal can also be used aselectrodes and will usually speedup the scavenging of the oxygen fromthe salt melt.

We have also found that when metals are to be tantallided or niobided,it is necessary to conduct the process in the absence of carbon andcarbon compounds because carbon forms very stable and refractorycarbides of tantalum or niobium on the surface of the base metalsthereby rendering it very difficult to penetrate these diffusionbarriers and form tantallided and niobided coating on the substrates. Wehave found that carbon can be removed from the fused salt bath byoperating it as a cell employing as a cathode, the-base metals such asnickel or iron, until the carbide coating is no longer formed on thesurface of the metal.

The base metals which can be tantallided or niobided, in accordance withthe process of this invention includes the metals having atomic numbersof 23-29, 41-46 and 73-79 inclusive. These metals are, for example,vanadium, chromium, manganese, iron, cobalt, nickel, copper, niobium,molybdenum, technetium, ruthenium, rhodium, palladium, tantalum,tungsten, rhenium, iridium, platinum and gold. Alloys of these metalswith each other or alloys containing these metals as the majorconstituent, that is, over 50 mole percent, alloyed with other metals asa minor constituent, that is less than 50 mole percent, can also bemetallided in accordance with our process, providing the melting pointof the resulting alloy is not lower than the temperature at which theused salt bath is being operated.

In order to produce a reasonably fast diffusion rate and to insure thefusion of the metal into the base metal to form a tantallide or niobidecoating, we have found it desirable to operate our process at atemperature no lower than about 850 C. It is usually preferred tooperate at temperatures of from 900 C. to 1100 C.

When an electrical circuit is formed external to the fused salt bath byjoining the anode to most of the previously listed base metal cathodesby means of a conductor, an electric current will flow through thecircuit without any applied electromotive force. The anode acts bydissolving in the fused salt bath to produce electrons and the metalions. The electrons flow through the external circuit formed by theconductor, and the metal ions migrate through the fused salt bath to thebase metal cathode to be metallided, where the electrons discharge themetal ions, forming a metallide coating. The amount of current can bemeasured with an ammeter which enables one to readily calculate theamount of metal being deposited on the base metal cathode and beingconverted to the metallided layer. Knowing the area of the article beingplated, it is possible to calculate the thickness of the metallidecoating formed, thereby permitting accurate control of the process toobtain any desired thickness of the metallide layer.

Although the process operates on most of the substrates listed abovevery satisfactorily without impressing any additional electromotiveforce on the electrical circuit, we have found it possible to apply asmall voltage when it is desired to obtain constant current densitiesduring the reaction and to increase the deposition rate of the metalbeing deposited without exceeding the diffusion rate of the metal intothe base metal cathode. The additional should not exceed 1.0 volt andpreferably should fall between 0.1 to 0.5 volt.

When tantalum is being diffused into niobium, or vanadium it is alwaysnecessary to apply a small external cathodic potential to these metalssince they are slightly more reactive than tantalum. The same is truefor the niobiding of vanadium.

When it is desirable to apply additional voltage to the circuit in orderto shorten the time of operation, the total current density should notexceed 10 amperes/cm. At current densities above 10 amperes/cm, thetantalum or niobium deposition rate exceeds the diffusion rate and thebase metal cathode becomes coated with a plate of tantalum or niobium.

Since the diffusion rate of tantalum and niobium into the cathodearticle varies from one material to another, with temperature, and withthe thickness of the coating being formed, there is always a variationin the upper limits of the current densities that may be employed. Therefore, the deposition rate of the iding agent must always be adjustedso as not to exceed the diffusion rate of the iding agent into thesubstrate material if high efliciency and high quality diffusioncoatings are to be obtained. The maximum current density for goodtantalliding or niobiding is 10 amperes/cmfi, when operating within thepreferred temperature ranges of this disclosure. Higher currentdensities can sometimes be used to form coatings with tantalum andniobium but in addition to the formation of a metallide coating, platingof the iding agent occurs over the diffusion layer.

Very low current densities (0.01-0.l amp/cm?) are often employed whendiffusion rates are correspondingly low, and when very dilute surfacesolutions or very thin coatings are desired. Often the composition ofthe diffusion coating can be changed by varying the current density,producing under one condition a composition suitable for one applicationand under another condition a composition suitable for anotherapplication. Generally, however, current densities to form good qualitytantallide or niobide coatings fall between 0.2 and 4.0 amperes per cm.for the preferred temperature ranges of this disclosure.

If an applied E.M.F. is used, the source, for example, a

battery or other source of direct current, should be connected in serieswith the external circuit so that the negative terminal is connected tothe external circuit, terminating at the metal being metallided and thepositive terminal is connected to the external circuit terminating atthe metal anode. In this way, the voltages of both sources arealgebraically additive.

As will be readily apparent to those skilled in the art, measuringinstruments such as voltmeters, ammeters, resistances, timers, etc., maybe included in the external circuit to aid in the control of theprocess.

Because the tough, adherent, corrosion resistant properties of thetantallide or niobide coatings are uniform over the entire treated area,the coated metal compositions prepared by our process have a widevariety of uses. They can be used to protect reaction vessels andapparatus from chemical attack, electro-chemical corrosion, and anodicoxidation, to make gears, bearings, and other articles reqiuring hard,wear-resistant surfaces, and to prevent corrosion at high temperatureson gas turbine material, heating elements etc. Other uses will bereadily apparent to those skilled in the art as well as othermodifications and variations of the present invention in light 'of theabove teachings.

In the specification and claims we use the term tantalide and niobide todesignate any solid solutions or alloys of tantalum and nobium and thebase regardless of whether the base metal does or does not form anintermetallic compound with tantalum and niobium in definitestoichiometric proportions which can be represented by a chemicalformula.

The following examples serve to further illustrate our invention. Allparts are by weight unless otherwise stated.

EXAMPLE I Into a Monel vessel (3%" in diameter x 12" deep) was placedlithium fluoride (1720 grams). The Monel vessel was placed in anelectric furnace. After, a nickel plated steel cover was attached whichcontained a water channel for cooling, 2 ports for electrodes andanother 2 ports for a thermocouple well and an argon sparge tube. Vacuumconnections were made to one of the electrode ports and the salt wasthen alternately evacuated and repressurized with argon three times atroom temperature, and then heated to 300 C. under argon at whichtemperature evacuation and repressurizing with argon was repeated threemore times; the salt was then heated under argon to melting (M.P.'=842C.). The salt temperature was raised to 900 C. at which point 78 gramsof potassium fluorotantalate salt (K TaF- was added through an electrodeport.

A A" diameter tantalum rod (anode) was immersed into the salt andclean-up of salt impurities was accomplished by immersing nickelscreens'(cathodes -50 cm. each) in the salt at 900 C. and electrolyzingat 4 amps for 15 minutes. After 10 amp-hours of electrolysis coulombicetficiencies (based on reduction of Ta+ to Ta) approaching 100% wererealized.

EXAMPLE II Strips of nickel (40 cm. in area) were then tantallided at1050 C. and at voltages ranging from +0.3 to +0.9 volt (anode polarity)with the following results:

TABLE I Current Wt. Coulombic Time density gain efficiency (mins(amps/cm!) (grams) (percent) EXAMPLE III A sample of expanded nickelscreen was next tantallided at 1050 C. with the following results:

TABLE II Volts Current (anode density polarity) (amps/cm!) 6 EXAMPLE IVFour other expanded nickel screens at 1050 C. with the followingresults:

were tantallided The voltage (anode polarity) remained negative at alltimes indicating that diffusion of Ta was exceeding the deposition rate.

The expanded nickel screen containing 6.5 mg. Ta/cm. was incorporated asan air cathode current collector in a phosphoric acid matrix fuel celloperating at 15 0 C. The tantallided screen exhibited excellentelectrochemical corrosion resistance for the 111 hour test period as acurrent collector material, i.e. the screen performed as well as a goldscreen current collector in the same type fuel cell.

The three remaining tantallided screens containing 7.1, 12.0 and 15.0mg. Ta/cm. were incorporated as air cathode current collectors in asulfonic acid solid polymer electrolyte fuel cell operating at 60 C. Thetantallided screens exhibited excellent electrochemical corrosionresistance for 450 hours as current collector materials, i.e., thescreens performed as well as a gold screen current collector int he sametype fuel cell.

EXAMPLE V Strips of 1020 mild steel, 4340 tool steel and Carpenter 20Cb-3 (50 cm. in area) were tantillided at 1080" C. and -.050 to +0.1volts (anode polarity), with the following results.

TABLE IV Current Coulombie Time density mg. efiiciency Material (mins.)(amps/cm?) Ta/cm. (percent) The tantallide diffusion coatings formedwith the 1020 mild steel and 4340 tool steel were shown to bepredominately a tantalum carbide; the tantallide diffusion coatingsformed with the Carpenter 20 Cb-3 was checked for Ta/ Fe alloy formationwhich was found to be present, along with the other components of theCarpenter 20 Cb-3.

Samples of tantallided 1020 mild steel, 4340 tool steel, and Carpenter20 Cb-3 stainless steel were subjected to anodic electrochemicalcorrosion in 1.5 N H 80 at 80 C., the tantalided 1020 and 4340 steelshad marginal corrosion resistance, but the tantallided Carpenter 20 Cb-3exhibited excellent corrosion resistance comparable to the cororsionresistance of tantallided expanded nickel screens from Example III.

I EXAMPLE VI Strips of cupron (45% Ni/55% Cu alloy) and molybdenum (30cm. in area) were tantallided at 1050 C. and -0.070 to +0.600 volt(anode polarity) with the following results:

I 7 EXAMPLE vn A niobium anode can be substituted for the tantalum anodeand KzNbFq for the KgTaFq in the lithium fluoride bath and the celloperated as given in the above ex amples to give niobiumdiffusion'coatings on the various base metal cathodes discussed above.

It will, of course, be apparent'to those skilled in the art thatconditions other than those set forth in the above examples can beemployed in the process of this invention without departing from thescope thereof.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A method of forming a tantallide or niobide difiusion coating on ametal composition having a melting point greater than 900 C., at least50 mole percent of said metal composition being at least one of themetals selected from the class consisting of metals whose atomic numbersare 23-29, 41-46, and 73-79, said method comprising,

(1) forming an electric cell containing said metal composition as thecathode, joined through an external electrical circuit to a tantalum orniobium anode and a fused salt electrolyte composed essentially of amember of the class consisting of lithium fluoride or sodium fluoride,mixtures thereof, mixtures of any of magnesium calcium, strontium andbarium fluo rides of the alkaline earth fluoride group, or mixtures ofthe said lithium or sodium fluoride with any of the above named alkalineearth fluorides, and from 0.01- mole percent of tantalum or niobiumfluorides, said electrolyte being maintained at a temperature of atleast 900 C. but below the melting point of said metal composition inthe substantial absence of oxygen,

(2) controlling the current flowing in said electric cell so that thecurrent density of the cathode does not exceed amperes/cm. during theformation of the tantallide or niobium diffusion coating, and

(3) interrupting the flow of electrical current after the desiredthickness of the tantallide or niobium difiusion coating is formed onthe metal composition.

2. The process of claim 1 wherein the absence of oxygen is obtained byuse of a vacuum.

3. The process as claimed in claim 1 which is also conducted in thesubstantial absence of carbon.

4. The process as claimed in claim 1 wherein the absence of oxygen isobtained by allowing tantalum or niobium metal to be in contact with thefused electrolyte prior to carrying out the process until the oxygen hasbeen depleted from the electrolyte bath.

5. The method of claim 1 wherein the metal composition is nickel. v

6. The method of claim 1 wherein the metal composition is cobalt.

7. The method of claim 1 wherein the metal composition is an alloy ofnickel and cobalt.

8. The method of claim 1 wherein the metal composition is an alloy ofnickel and copper.

9. The method of claim 1 wherein the metal composition is iron.

10. The method of claim 1 wherein the metal composi tion is anystainless steel alloy.

11. The method of claim 1 wherein the metal composition is molybdenum.

References Cited UNITED STATES PATENTS 3,514,272 5/1970 Cook 291943,489,540 1/1970 Cook 29194 3,489,539 1/1970 Cook 29-194 3,479,15911/1969 Cook 29194 3,479,158 11/1964 Cook 29-194 OTHER REFERENCES Tableof Periodic Properties of the Elements, E. H. Sargent & Co.

TA-HSUNG TUNG, Primary Examiner R. L. ANDREWS, Assistant Examiner

